High precision displacement measuring interferometer based on the active modulation index control method

•A compact, and high-precision laser-diode-based frequency-modulated interferometer is discussed.•A modulated laser diode source by directly coupling current modulation to the laser is proposed.•A high modulation frequency of 30 MHz for the laser diode to improve the measurement speed of the interferometer is implemented.•The modulation excursion is controlled to improve the measurement accuracy and range of the interferometer.•A multi-harmonic detection system is employed to obtain the highest SNR pair of harmonics and alternation between harmonics when a harmonic signal fades owing to the unbalanced length change. This study proposes a simple, high-precision, and wide-measurement-range displacement measuring interferometer. A frequency-modulated laser diode (LD) source at 30 MHz was employed for the interferometer. The interference signal comprised a series of harmonics of the modulation frequency. Normally, the signal-to-noise ratio (SNR) of harmonics changes rapidly because of the unbalanced length between the two arms of the interferometer; hence, the measurement accuracy and range were limited. In this study, the SNR of the harmonics was improved by controlling the modulation excursion of the LD. Moreover, all harmonics with high SNR in a specific measurement range can be detected simultaneously using lock-in amplifiers (LIAs). Therefore, the displacement was determined using the pair of harmonics with the best SNR from the detected harmonics. To verify the measurement accuracy, the proposed interferometer was utilised to measure the nanoscale displacement induced by a piezo actuator (PZT) with sinusoidal and triangular shapes.